I've always been told not to give any fluids until the kid hasn't barfed for an hour, but ds keeps heaving and at this point is just throwing up teeny amounts of mucous and bile.
Once he's gone an hour or so without dry heaving, we will very slowly reintroduce fluids. typically room temperature water or gatorade. One tsp of liquid every 5 minutes. if they keep it down for an hour so or, we'll increase the amount to 2-3 tsps every 5 minutes. Our dr always told us it's the quantity or weight of liquid that reactivates the vomiting. if you give small enough doses, the stomach won't get any weight from the liquid, and the greater the chance of no vomiting, hence the absorption of the liquid, which is what you need to prevent dehydration.
I'm certainly no expert, but that's what we do. Then, after a few hours, one they ask for food, we give them 1/2 cracker. If he keeps that down for a while, we'll give the other half. Eventually we work up to chicken broth, jello, etc. An ER doctor once told us to make ramen noodles.